The present invention generally relates to catalysts and methods for alkane or alkene dehydrogenation and specifically, to Ni-containing catalysts and methods for oxidative dehydrogenation of alkanes or alkenes. The invention particularly relates, in preferred embodiments, to Ni oxide/mixed-metal oxide catalysts and methods for oxidative dehydrogenation of alkanes or alkenes, and especially of C2 to C4 alkanes, and particularly, for oxidative dehydrogenation of ethane to ethylene.
Ethylene can be produced by thermal cracking of hydrocarbons, by non-oxidative dehydrogenation of ethane, or by oxidative dehydrogenation of ethane (ODHE). The latter process is attractive for many reasons. For example, compared to thermal cracking, high ethane conversion can be achieved at relatively low temperatures (about 400xc2x0 C. or below). Unlike thermal cracking, catalytic ODHE is exothermic, requiring no additional heat to sustain the reaction. In contrast to catalytic non-oxidative dehydrogenation, catalyst deactivation by coke formation is relatively minimal in ODHE because of the presence of oxidant (e.g., molecular oxygen) in the reactor feed. Other alkanes can be similarly oxidatively dehydrogenated to the corresponding alkene.
Thorsteinson and coworkers have disclosed useful low-temperature ODHE catalysts comprising mixed oxides of molybdenum, vanadium, and a third transition metal. E. M Thorsteinson et al., xe2x80x9cThe Oxidative Dehydrogenation of Ethane over Catalyst Containing Mixed Oxide of Molybdenum and Vanadium,xe2x80x9d 52 J. Catalysis 116-32 (1978). More recent studies examined families of alumina-supported vanadium-containing oxide catalysts, MV and MVSb, where M is Ni, Co, Bi, and Sn. R. Juarez Lopez et al., xe2x80x9cOxidative Dehydrogenation of Ethane on Supported Vanadium-Containing Oxides,xe2x80x9d 124 Applied Catalysis A: General 281-96 (1995). Baharadwaj et al. disclose oxidative dehydrogenation of ethane and other alkanes using a catalysts of Pt, Rh, Ni or Pt/Au supported on alumina or zirconia. See PCT Patent Application WO 96/33149. U.S. Pat. No. 5,439,859 to Durante et al. discloses the use of reduced, sulfided nickel crystallites on siliceous supports for dehydrogenation and successive oxidation of alkanes. Schuurmnan and coworkers describe unsupported iron, cobalt and nickel oxide catalysts that are active in ODHE. Y. Schuurmnan et al., xe2x80x9cLow Temperature Oxidative Dehydrogenation of Ethane over Catalysts Based on Group VIII Metals,xe2x80x9d 163 Applied Catalysis A: General 227-35 (1997). Other investigators have also considered the use of nickel or nickel oxide as catalysts or catalyst components for oxidative dehydrogenation. See, for example, Ducarme et al., xe2x80x9cLow Temperature Oxidative Dehydrogenation of Ethane over Ni-based Catalystsxe2x80x9d, 23 Catalysis Letters 97-101 (1994); U.S. Pat. No. 3,670,044 to Drehman et al.; U.S. Pat. No. 4,613,715 to Haskell; U.S. Pat. No. 5,723,707 to Heyse et al; U.S. Pat. No. 5,376,613 to Dellinger et al.; U.S. Pat. No. 4,070,413 to Imai et al; U.S. Pat. No. 4,250,346 to Young et al.; and U.S. Pat. No. 5,162,578 to McCain et al.
Although nickel-containing catalysts are known in the art for alkane dehydrogenation reactions, none of the known nickel-containing catalysts have been particularly attractive for commercial applicationsxe2x80x94primarily due to relatively low conversion and/or selectivity. Hence, a need exists for new, industrially suitable catalysts and methods having improved performance characteristics (e.g., conversion and selectivity) for the oxidative dehydrogenation of alkanes.
It is therefore an object of the present invention to provide for new, industrially suitable catalysts for oxidative dehydrogenation of alkanes to the corresponding alkenes.
Briefly, therefore, the invention is directed to methods for preparing an alkene, and preferably a C2 to C4 alkene, such as ethylene, from the corresponding alkane, such as ethane. In general, the method comprises providing the alkane (or substituted alkane), and preferably the C2 to C4 alkane (or substituted C2 to C4 alkane) and an oxidant to a reaction zone containing a catalyst, and dehydrogenating the alkane to form the corresponding alkene. The oxidant is preferably a gaseous oxidant such as molecular oxygen, and is preferably provided, for example, as oxygen gas, air, diluted air or enriched air. The alkane is preferably oxidatively dehydrogenated. The reaction temperature is preferably controlled, during the dehydrogenation reaction, to be less than about 325xc2x0 C., and preferably less than about 300xc2x0 C.
The catalyst comprises, in one embodiment, (i) a major component consisting essentially of Ni, a Ni oxide, a Ni salt, or mixtures thereof, and (ii) one or more minor components consisting essentially of an element or compound selected from the group consisting of Ti, Ta, Nb, Co, Hf, W, Y, Zn, Zr, Al, oxides thereof and salts thereof, or mixtures of such elements or compounds. The catalyst preferably comprises Ni oxide and one or more of Ti oxide, Nb oxide, Ta oxide, Co oxide or Zr oxide.
In another embodiment, the catalyst comprises a compound having the formula I,
NixAaBbCcOdxe2x80x83xe2x80x83(I),
where A is an element selected from the group consisting of Ti, Ta, Nb, Hf, W, Y, Zn, Zr, Al, and mixtures of two or more thereof, B is an element selected from the group consisting of a lanthanide element, a group IIIA element, a group VA element, a group VIA element, a group IIIB element, a group IVB element, a group VB element, a group VIB element, and mixtures of two or more thereof, C is an alkali metal, an alkaline earth metal or mixtures thereof, x is a number ranging from about 0.1 to about 0.96, a is a number ranging from about 0.04 to about 0.8, b is a number ranging from 0 to about 0.5, c is a number ranging from 0 to about 0.5, and d is a number that satisfies valence requirements.
In a further embodiment, the catalyst comprises a compound having the formula (II)
NixTijTakNblLa*SbrSnsBitCauKvMgwOdxe2x80x83xe2x80x83(II),
where La* is one or more lanthanide series elements selected from the group consisting of Lam, Cen, Pro, Ndp, Smq, x is a number ranging from about 0.1 to about 0.96, j, k and l are each numbers ranging from 0 to about 0.8 and the sum of (j+k+l) is at least about 0.04, m, n, o, p, q, r, s and t are each numbers ranging from 0 to about 0. 1, and the sum of (m+n+o+p+q+r+s+t) is at least about 0.005, u, v and w are each numbers ranging from 0 to about 0. 1, and d is a number that satisfies valence requirements.
In still another embodiment, the catalyst comprises (i) a Ni oxide, and (ii) an oxide of an element selected from the group consisting of Ti, Ta, Nb, Co, Hf, W, Y, Zn, Zr, and Al, and the alkane is dehydrogenated to form the corresponding alkene with an alkane conversion of at least about 10% and an alkene selectivity of at least about 70%. Ethane conversion is preferably at least about 15% and more preferably at least about 20%. Ethylene selectivity is, in combination with any of the preferred conversion values, preferably at least about 80%, and more preferably at least about 90%.
In one embodiment, the catalyst is a calcination product of a catalyst precursor composition comprising (i) Ni, a Ni oxide, a Ni salt or mixtures thereof, and (ii) an element or compound selected from the group consisting of Ti, Ta, Nb, Co, Hf, W, Y, Zn, Zr, Al, oxides thereof and salts thereof, or mixtures of such elements or compounds.
In yet another embodiment, the alkane is co-fed to a reaction zone with the corresponding alkene, such that the alkane is dehydrogenated to form the alkene in a reaction zone comprising the corresponding alkene in a molar concentration of at least about 5%, relative to total moles of hydrocarbon. The alkane conversion in such embodiment is preferably at least about 5%, and the alkene selectivity is preferably at least about 50%. In a preferred approach, the alkane dehydrogenation is effected in a multi-stage reactor, such that the alkane (or substituted C2 to C4 alkane) and gaseous oxidant are fed to a first reaction zone containing the catalyst, the alkane is dehydrogenated therein to form the corresponding alkene, the product stream comprising the corresponding alkene and unreacted alkane are exhausted from the first reaction zone and then fed to a second reaction zone, together with additional, supplemental gaseous oxidant, and the alkane is dehydrogenated to form the corresponding alkene in the second reaction zone.
The invention also directed to nickel-containing mixed-metal oxide compositions and catalysts, as characterized above, and to methods for preparing the same.
Such catalysts and methods have advantageous performance characteristics for oxidative dehydrogenation of alkanes to their corresponding alkene, and particularly for dehydrogenation of unsubstituted or substituted C2 to C4 alkanes to the corresponding alkene(s). The conversion, selectivity, space velocity, catalyst stability and reaction temperature for oxydehydrogenation of ethane to ethylene are particularly attractive.
Other features, objects and advantages of the present invention will be in part apparent to those skilled in art and in part pointed out hereinafter. All references cited in the instant specification are incorporated by reference for all purposes. Moreover, as the patent and non-patent literature relating to the subject matter disclosed and/or claimed herein is substantial, many relevant references are available to a skilled artisan that will provide further instruction with respect to such subject matter.